In tribute to the late Sir Terry Pratchett (12 March 2015), many websites have added the message “GNU Terry Pratchett” into the html header “X-Clacks-Overhead”.

…of course this personal website is doing the same.

If you want to know if a website is sending this message, you can use a browser plugin/extension to display the X-Clacks-Overhead in Clacks Semaphore, and you will see a similar icon in your browser.
The plugins are: